1. Field of the Invention
The invention relates to a method of storing image information in coded form representing a series of images which succeed one another in time.
The invention also relates to a diagnostic system using the method.
The invention further relates to an image coding and recording unit and an image information retrieval unit for use in such a system.
2. Description of the Related Art
The storage of image information representing moving images or pictures is known from the SMPTE Journal, Volume 102, No. Jul. 7, 1993, pp. 612-615. Such a method can be used, for example, in so-called cardiovascular diagnosis where cardiac function is studied by introducing a contrast medium into a blood vessel of the heart and subsequently malting a series of radiographs to record how this contrast medium propagates in the blood vessels of the heart as a function of time. The series of images obtained by means of the radiographs are converted into coded data streams which are subsequently recorded on a suitable record carrier, for example in the form of a magnetic or optical tape or disc. The image information thus recorded is read out and displayed at a later instant to analyze the cardiac function. This analysis is effected y examining moving images resulting from displaying the series of images in succession and if close scrutiny of the one or more individual images of the series is required, they are displayed as stationary or still images. For this purpose it is desirable that each of the individual images can be displayed selectively as a still image, enabling both preceding images in the series and subsequent images to be selected. For a correct diagnosis it is desirable that the images recovered from the coded information read from the record carrier are of the same quality as the originally recorded images. This means that the amount of information to be read out per image is large and, consequently, the data rate (amount of information per unit of time) with which the information is read should be high. This high data rate (hereinafter also referred to as xe2x80x9cread velocityxe2x80x9d) imposes stringent requirements on the read devices by means of which the image information is read.
It is an object of the invention to provide a method by means of which, on the one hand, the image information is recorded in such a manner that a rapid and reliable analysis of the image information is possible and for which, on the other hand, the data rate with which the image information is read is limited.
This object is achieved by means of a method of recording in coded form on a record carrier image information relating to moving images for the purpose of further analysis at a later instant, the image information representing moving images in the form of a series of images which succeed one another in time, said series of images being converted into a first data stream in which each of the series of succeeding images is represented, which first data stream is subsequently recorded on the record carrier, characterized in that the series of images is converted into a second data stream in which each image of the series is represented likewise, the average amount of information per image in the first data stream being smaller than in the second data stream, and the second data stream being recorded on the record carrier together with the first data stream.
The method in accordance with the invention advantageously utilizes the fact that other requirements are imposed on the reproduction of moving images than on the reproduction of still images. For example, for the reproduction of moving images it is essential that the rate at which the successive images of the series are reproduced is high enough to achieve an impression of non-intermittent motion. In general, the image quality is considered to be less important for moving images than for still images. Conversely, the rate at which successive images can be reproduced is less important for the reproduction of still images. With a given limited read velocity of a read device the recording of the two data streams has enabled the moving images to be coded by a coding method which to an optimum extent meets the requirements imposed on the reproduction of moving images, the recording of the second data stream yet enabling each individual image to be reproduced with a high quality. The fact that the time required to read out the information relating to the still image to be displayed is longer than the time between successive images during the reproduction of moving images does not pose a problem provided that it remains within acceptable limits.
It is to be noted that JP-A-1-243184 discloses a device for displaying image information. Moving image information in the form of a series of time sequential images are stored in irreversibly compressed form in a RAM. In addition, for some of the images of a series of successive images representing the moving pictures a reversibly compressed image is stored in the RAM. For the reproduction of moving the irreversibly compressed images of a series of successive images are read out, expanded and subsequently applied to an display device. For the reproduction of a still image a reversibly compressed image is read out, expanded and applied to the display device. With the disclosed device a reversibly compressed image is available only for a very limited number of images of the series of successive images representing the moving pictures and therefore the device is not suitable for the thorough analysis of image information as required, for example, in cardiac function analysis. Indeed, for this analysis a high-quality still image is required for each of the images of the series of images forming the moving images.
An embodiment of the method is characterized in that reference information is recorded on the record carrier, which reference information is indicative of the locations where corresponding images are situated in the first and the second data stream.
This embodiment has the advantage that during the analysis which follows an interruption in the reproduction of the moving image the associated high-quality coded image in the second data stream can be located rapidly on the basis of the reference information.
An embodiment of the method is characterized in that the conversion of the series of images in the first data stream is of a type in which images of the series are coded by coding differences between the relevant image and another image of the series, each image in the second data stream being represented by a separately coded image which has been coded independently of the image information of other images of the series.
This embodiment has the advantage that as result of the redundance in the successive images a high degree of compression and, consequently, a low required read velocity can be obtained while preserving a satisfactory image quality. In this case the disadvantage of such compression techniques that arbitrary images cannot simply be retrieved does not play a part owing to the availability of the second data stream. Another disadvantage that the reproduced images based on the first data stream exhibit distortion, i.e. quantization noise, is also of minor importance because such imperfections are considered to be less annoying in the case of moving images and because any individual image can be reproduced without distortion owing to the presence of the second data stream. A particularly suitable image coding for obtaining the first data stream is the so-called MPEG coding.
A further embodiment of the method is characterized in that the series of images is also converted into a third data stream in which each image of the series is represented, the average amount of information per image in the third data stream being smaller than in the first data stream.
The record carrier obtained by means of this embodiment enables moving images to be reproduced smoothly by means comparatively simple and, consequently, cheap read devices which read the third data stream with a low read velocity. The quality of the images reproduced on the basis of the third data stream is of a lower grade than that of images based the first and the second data stream but often the interpretation of the images does not require image reproduction with the highest quality. In many cases it is adequate to reproduce images with a lower quality. For example, when the cardiac function is discussed with another physician it is often adequate to display the image information with a lower resolution.
Since the resulting record carrier permits of high quality reproduction by means of advanced read devices and lower quality reproduction by means of cheaper read devices the use of sophisticated expensive read equipment can be restricted to those cases requiring the high image quality.
Moreover, it is to be noted that the record carrier area occupied by the lower quality image information is small in comparison with the area needed for the storage of the high quality images, so that the total amount of image information which can be recorded on a record carrier is reduced only slightly as result of the storage of the lower quality images.
A further embodiment of the method is characterized in that the series of images is also converted into a fourth data stream, each image being separately coded independently of the image content of the other images of the series, the average amount of information per image in the fourth data stream being smaller than in the second data stream, and the fourth data stream being recorded on the record carrier together with the first, the second and the third data stream.
This enables both moving images and still images to be reproduced by means of less expensive read equipment in a similar way as by means of sophisticated read equipment.
It is particularly attractive to code and record the lower quality moving images and the associated still images in accordance with a format which complies with the so-called CD-I standard. This enables the lower quality images to be reproduced by means of a cheap mass product developed for the consumer market.
In addition to the audiovisual information a CD-I should contain an application program which controls the access to the stored information during the read process. By adapting this application program so as to prevent the CD-I player from reading information not coded in accordance with the CD-I standard the recording of the higher quality images on a CD-I will have no consequences for the read-out of the CD-I disc by a CD-I player.
The read device for reading the higher resolution image information may be provided with a control system which enables the files with higher resolution image information to be read.
During an examination of the cardiac function the image information is generally obtained in a number of so-called runs. The length of a run is determined by the time required by the contrast medium to propagate in the blood vessels of the heart. This time is of the order of magnitude of ten seconds.
An embodiment of the method which is particularly suitable for use in the above-mentioned conditions is characterized in that at least the first and the second data stream, which have been derived from a continuous series of images, are recorded in a continuous part of the record carrier. Since with this embodiment the information relating to the moving images as well as the information relating to the still images for one run are recorded together the search time upon a change from moving image reproduction to still image reproduction (or vice versa) is short.
A further embodiment is characterized in that for each continuous series of images a first part of the second data stream is recorded in such a way that it directly precedes the first data stream and a second part of the second data stream is recorded in such a way that it directly follows the first data stream.
This embodiment has the advantage that if it is desired to read a still image during the read-out of moving images the distance to the new the read location will only be minimal, which results in a short access time.
The method in accordance with the invention can be used advantageously in a diagnostic system comprising
means for acquiring image information relating to an object to be examined, which image information comprises a series of images which succeed one another in time,
an image coding and recording unit comprising first coding means for converting the series of images which succeed one another in time into a first data stream in which each of the series of succeeding images is represented, and second coding means for converting the image information into a second data stream in which each image of the series is represented likewise, the average amount of information per image in the first data stream being smaller than in the second data stream, and the image coding and recording unit further comprising a recording device for recording the first and the second data stream on one and the same record carrier,
an image information retrieval unit of a first type comprising read means for reading the record carrier with a first read velocity, first decoding means for recovering successive images from the first data stream, second decoding means for recovering individual images from the second data stream, and image output means for supplying recovered images to an image reproducing device, control means for controlling the read means, the first and the second decoding means and the image output means, which control means are adapted to achieve that in a first mode a part of the recorded first data stream is read, a series of images corresponding to the read-out part is recovered by the first decoding means and said recovered images are supplied by the image output means, and the control means are further adapted to achieve that in a second mode a selected separately coded image of the second data stream is read, an image corresponding thereto is recovered from said coded image by the second decoding means and said recovered image is supplied by the image output means.